1. Field of the Invention
This invention relates to new vitreous carbon compositions derived from the addition of a phenol-furfuraldehyde Novolac to a phenol-aldehyde "resol" resin. More specifically, it relates to the production of vitreous carbon in large, molded sizes, particularly large thin plates produced by appropriately compressed and molded shaped articles which are carbonized and then graphitized to excellent vitreous carbon with low percentages of failures or rejects. Still more specifically, it relates to a process for making such vitreous carbon products.
2. State of the Prior Art
The extreme inertness and non-porosity of vitreous carbon qualifies it as an important and useful material of construction for use in a number of applications and in various industries. For research and development work this material has been used in fabricating beakers, basins, boats, reaction tubes, etc., and for extensive use in the processing of semiconductors, fluoride laser materials, zone refining of metals, zone refining of chemicals, biomedical applications, fuel cell electrodes, etc.
However, industrial applications of vitreous carbon have been made only in recent years. Since vitreous carbon is not wet by a wide range of metals including zinc, silver, copper, tin, lead, aluminum, gold, platinum and others, it has found application in the processing of some of these metals and their alloys, for example, in the dehydrogenation of molten aluminum with chlorine gas. Dip pipes of this material for corrosive liquids have also been successfully used.
The growth of industrial applications for this material has been restricted in large measure by the inability to produce properly cured moldings or extrusions in the required shapes and by use of conventional molds or dies at conventional rates and reasonable cost. There has been difficulty in making large thin plates suitable for use in fuel cells.
A "resol" resin is the resinous reaction product of a phenol and an aldehyde which has been condensed (reacted) only to a stage where it still melts when heated and is still soluble in acetone, and the resin still has sufficient residual reactivity that it may be cured by heat without the addition of a curing agent to an insoluble and infusible condition. A resol resin is also known as an "A" stage phenolic resin, or also as a "single stage" resin, because it is curable without the addition of any crosslinking agent.
The "resol" resin is prepared by using the aldehyde in a molar proportion greater than 1-1 with the phenol. Since sufficient aldehyde is already present to give a cure to the insoluble, infusible state, there is no need to add a curing agent such as hexa for final curing. However, in preparing the resol resin, it may be desirable to add a small amount of hexa to obtain a harder and more easily grindable resol. For example, 0.005 to 0.03 mole, preferably 0.01 mole of hexa per mole of phenol is advantageous for this purpose. In any case, the amount of hexa used in preparing the resol is not calculated in the amount which may be subsequently added to aid in the curing of the Novolac-resol mixture.
In contrast, a "Novolac" resin is one prepared with a deficiency in aldehyde so that it may not be cured unless a curing agent such as hexa is added. Therefore, a "Novolac" resin may be defined as the resinous reaction product of a phenol and an aldehyde that, for all practical purposes, does not harden or convert to an insoluble, infusible condition upon heating, but remains soluble and fusible.
In starting with resin particles as described in said pending application, it has been found particularly important that the Novolac is a phenolfurfuraldehyde resin. When the Novolac is one using furfuraldehyde, pressures are suitable for compressing the resin mixture into the desired shapes for molding which are not suitable for Novolacs made with other aldehydes. As shown hereinafter, this becomes evident in the number of rejects produced when the molded objects are carburized to vitreous carbon.
In the parent applications, Ser. Nos. 182,755, 356,893 and 502,181, a number of references were cited with respect to coreaction of Novolac and resol resins. These include U.S. Pat. Nos. 3,998,906, 3,927,140, 3,879,338, 3,410,718, Japanese Patent No. 53-75294 and British Patent No. 1,090,029. While these references relate to curing various mixtures of resins, none of these pertain to water removal from the various mixtures so as to produce a moldable composition, particularly one which may be subsequently mixed with a carbonaceous filler such as graphite for further processing into vitreous carbon products.